Why does my electronic digital calendar change time after power failure?

The electronic calendar remembers the time and configuration according to the battery. When you connect the power supply, it will automatically charge the small battery. When the power supply is disconnected, the memory battery will remain configured and stop displaying the electronic perpetual calendar to save power. Therefore, when the power failure time exceeds the working time of the memory battery, it will return to the initial setting of the electronic perpetual calendar.